Embodiments of the inventive subject matter generally relate to the field of communications networks, and, more particularly, to intermodulation interference resulting from signals transmitted via a communications network.
Intermodulation (IM) or intermodulation interference is caused when two or more different frequencies are transmitted in a system that has nonlinearities. The nonlinear effect of the system can form additional signals (e.g., IM interference signals) at frequencies that are different from the originally transmitted frequencies. IM interference may occur at frequencies other than known harmonic multiples of the transmitted frequencies. Nonlinearities in the communications system or in devices coupled to the communications system may cause unpredictable IM interference. For example, nonlinearities can create unwanted spurious signals due to intermodulation between a transmitted signal with other nonlinear devices that are coupled to the communications system.
The spurious signals can cause interference for the communications system, or even for other communications systems that operate in other frequency bands. Out of band emissions due to intermodulation may cause interference for devices operating in a different frequency range than the originally transmitted signals. An example of this may include IM interference caused by originally transmitted signals in a powerline communications (PLC) network, where the IM interference overlaps with FM radio signals at frequencies not typically used by the PLC network. Because intermodulation interference caused by devices in a communications system may interfere with protected frequencies, a regulatory agency may limit power in the communications system. However, it may be difficult to predict which transmitted frequencies and power levels will produce IM interference.